In recent years, the amount of data processed in personal computers (abbreviated to “PCs” hereinafter) or measuring instruments has been abruptly increased, and the integration degree or density of multi-layered wiring boards or semiconductors, which are important members in each of the instruments, has been becoming high, and electric parts have been becoming large in size.
In such a background, the heat quantity generated inside a PC has also been becoming larger than that of any conventional PC. In the operation thereof, troubles (such as malfunction and operation inability) and other problems have been caused.
In order to solve the problem, individual PC manufactures take measures as follows: in order to cool the inside rapidly, a cooling fan higher in performance than conventional fans is set; or in order to improve the heat-radiating property of a heat generation body, a soft heat radiation material is interposed between the heat generation member and a heat radiation plate to improve the contact between the members, thereby giving a better heat-radiating property. However, the improvement in the performance of the cooling fan results in making the PC size large, and causes an increase of noises and a rise in costs.
Thus, attention has been paid to a heat radiation member used between the heat generation body and the heat radiation plate, and to a heat radiation material which is excellent in heat-radiating property, smaller in thickness, and small in change in physical properties which follows a change in PC inside temperature.
As a typical heat radiation member, a grease type material (material having stickiness) is used in the state that the material is interposed between a heat generation member and a heat radiation plate (such as a metallic plate) to improve the heat radiation property. However, according to this method, the stickiness of the grease causes a poor handleability, so as to result in a poor workability when the heat radiation device is fabricated. Moreover, with a temperature change, the viscosity of the grease is largely changed; thus, a change in the heat-radiating property has been become a problem.
As a workability-improved material of the grease type heat radiation material, attention has been paid to a heat radiation sheet having softness and surface tackiness (bonding function based on stickiness), and being stable in physical properties.
However, heat radiation sheets commercially available at present are poor in balance between problems against heat-radiating property (change in characteristics in accordance with the use temperature) and against handleability on the basis of insufficient strength, and sheet properties (softness and surface tackiness).
As a heat radiation metallic material having softness, indium made into a sheet form is used. However, the use thereof is limited within a partial scope since the metal is high in costs.
In such a situation, suggested are various composite materials that are low in costs and excellent in heat-radiating property and workability, and products worked therefrom. For example, Patent Document 1 discloses a heat-radiating resin shaped product wherein graphite powder is blended with a thermoplastic resin, and Patent Document 2 discloses a polyester resin composition containing graphite, carbon black or the like. Patent Document 3 discloses a rubbery composition into which artificial graphite having a particle diameter of 1 to 20 μm is incorporated, and Patent Document 4 discloses a composition wherein spherical graphite having a crystal plane interval of 0.330 to 0.340 nm is incorporated into a silicone rubber. Patent Document 5 describes a highly heat-radiating composite material characterized in that specified graphite particles are pressed/compressed in a solid, whereby the particles are lined in parallel to the surfaces of the composition, and a process for producing the material.
However, even a shaped product as described above, wherein specified graphite powder is dispersed at random or wherein graphite particles are pressed/compressed to be lined, is not sufficient yet in thermoconductivity.
Against this, Patent Document 6 discloses a heat-radiating shaped product wherein the c axis of the crystal structure of graphite powder is oriented perpendicularly to the heat radiation direction, and a process for producing the product.
Heat radiation sheets are each required to be excellent in sheet handleability in order to makes it easy to fabricate a heat radiation device therefrom. For the method of making better use of the sheet handleability, there is generated a need that the sheet is caused to have properties such as followability coping with irregularities, curves or other especial shapes of a heat generation body surface and a heat radiation member surface, or a stress relieving function or some other function. For example, in heat radiation from a display panel, or some other large area, a sheet-handling function is an important theme from the viewpoint of an improvement in productivity. Accordingly, functionalization of properties of a heat radiation sheet has been advancing, and it has been intensely desired to develop a sheet that is excellent in tackiness, softness, heat resistance and strength, and is high in heat-radiating property.
However, the heat-radiating shaped product disclosed in Patent Document 6 is insufficient in tackiness, softness, heat resistance and strength although the product is excellent in heat-radiating property. Thus, the product has a problem about the sheet handleability thereof.